The EDM process makes use of electroerosive power pulses applied between a workpiece and tool electrode spaced apart in juxtaposed relation across an electrode gap flooded generally with a dielectric liquid (e.g. kerosine or distilled water) which also serves to carry away the detritus of the electrical discharge machining process.
The tool electrode is generally formed with the desired configuration of the cavity or shape complementarily desired in the workpiece. A train of power pulses is then formed to create localized and discrete material removal discharges having a tendency to produce cumulatively overlapping craters in the workpiece surface; the total surface juxtaposed with the tool is thus machined uniformly over the parts thereof confronting the tool electrode and receives a configuration conforming to the shape of the tool electrode. In traveling-wire or wire-cut electrical discharge machining (TW-EDM), the tool electrode is formed by a continuous, axially-traveling elongate wire-like electrode and a two- or three-dimensional relative displacement between the wire and workpiece electrodes yields a desired shape or configuration in or on the workpiece.
During the machining operation, small metallic or conductive chips or particles removed from the electrode surface as well as other discharge products such as tar and gases are carried away by the liquid dielectric which floods the gap and is generally circulated therethrough while the tool electrode is advanced relative to the workpiece by a servo system designed to maintain a predetermined gap spacing or to approach the desired gap spacing as accurately as possible. The servo arrangement may also function to respond to gap short-circuiting and arcing conditions to retract the electrode relative to the workpiece thereby removing such conditions.
While various servo techniques have been practiced in the art, they are divided into two classes.
One includes the provision of a fixed reference voltage and derivation of an algebraic difference or combination of the reference voltage with a signal voltage drawn from the gap and representing a preselected gap variable. The result of this algebraic combination of the reference voltage and gap-sensing voltage is an output or control signal which operates a servomotor of the rotary or linear type to displace the movable electrode until this output signal has been reduced to zero or nullified. The direction of displacement of the servomotor is generally determined by the polarity of the output or difference signal and may be an "advance" or "retraction" signal to narrow or widen the gap spacing. The reference voltage is preset at a given value to correspond to a desired gap spacing and generally is fixed over its range of adjustment so that the difference signal and any amplified servomotor-drive signal assume a linear change in magnitude to the sensed gap variable. When these principles are used for electrical discharge machining, they are not always successful and cannot provide optimum results for several reasons including the fact that EDM parameters are essentially nonlinear in terms of the characteristics of the gap. For example, the gap is not a simple resistance between the electrodes which varies linearly in accordance with the width of the gap, because of changes in the composition of the coolant, the presence of particles, polarization, breakdown and other phenomena at the gap, intimately associated with the machining or processing. Furthermore, the response of the electrode system is seldom ideal so that an electrical command or control signal is not always converted linearly into a corresponding mechanical movement; the lack of correspondence of this type is a consequence of static and dynamic inertia, backlash and other factors present in the mechanical system. Hence, between the sensing of the gap condition and the displacement of the movable electrode to compensate and restore the desired gap condition, there is significant nonlinearity which analog sensing and control appears to multiply rather than decrease.
In another class of servo system, a gap detector is provided to derive a substantially continuous or analog signal varying as a function of a gap variable, generally the gap spacing and to feed it to a discriminator having a threshold value and producing an digital output depending upon its relationship to the threshold, i.e. a first digital state when the analog signal exceeds a threshold and a second digital state when the analog signal is below the threshold value. The digital states serve to represent "advance" and "retraction" signals and are employed to control the servomechanism and actuate the latter in a sense or direction determined by the prevalent digital state and to an extent determined by the duration of that state. The threshold may be constituted by one or more values preset on an empirical basis as corresponding to a desired gap spacing when the machining is carried out in an optimum mode or may be automatically adjusted upon monitoring the progress of a given machining operation.
As opposed to the first-mentioned systems, therefore, the servomechanism responsive to the digital state and reversible, i.e. operative to advance or retract the movable electrode in dependence upon the digital state prevalent at the moment, is capable of correcting the position of the movable electrode only upon the formation of an analog signal which exceeds or lies below the corresponding threshold value to create the digital state, the magnitude of which may be substantially independent of the analog output so that the system is free from the inconveniences of linear control arrangements for regulating nonlinear operations. Furthermore, the servo actuator (servo motor) receives only opposite-polarity pulses of variable width but fixed amplitude and relatively sharp flanks or wave fronts, and control pulses vary sharply between a pair of opposing states and thereby sharply energize the servo motor with either of two active conditions depending upon the magnitude of the analog signal derived from the gap and compared or discriminated with respect to the predetermined threshold value. This method of response and actuation provides rapid quenching of any short circuit or arc and rapid recovery of a normal gap such that only spark-type discharges are formed. The system also allows accelerated follow-up feed of the movable electrode after correction of any deficient operation, provides for rapid elimination of the effect of mechanical inertia, and otherwise ensures the rapid movement of the electrode in either direction.
In both types of EDM servo control described in the foregoing, however, the gap sensor or drive circuit acting on a servo actuator or motor for controlling the position of a movable electrode has hitherto been designed in effect to respond solely to changes in one or more gap variables which are extremely sensitive to and eventually dependent on instantaneous changes in gap state, other than the actual or effective gap size, which, due to the dynamic action of gap existents or formations, occur undefinably at one localized zone or another of the entire machining area defined by the confronting electrodes. This has been found to be especially disadvantageous when the servo actuator or motor is to operate in an "advance" or feed-forward mode to narrow the machining gap. The servo sensor or drive circuit may then respond to advance the movable electrode before machining discharges develop thoroughly over the entire machining area or when only a fraction of the machining area develops effective discharges. The result of this premature advance is an insufficient material removal at the previous machining position and the tendency to soon bring about a short-circuiting condition which must be cleared by a subsequent retraction of the movable electrode to follow immediately to return the latter at an optimum position; the retraction and advance cycle is done unnecessarily and must be repeated. Unavoidably, therefore, the operation becomes unstable and machining proceeds with only a limited efficiency.